<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text">
<input type="button" value="平方" onclick="f()">
<input type="button" value="求和" onclick="f1()">
<div>结果:</div>
<script>
  let i = document.querySelector("input");
  let d = document.querySelector("div");
  function f1(){
    //当字符串进行-*/运算时会自动将字符串转成数值
    // d.innerText = "结果:"+(i.value*1+i.value*1);
    d.innerText = "结果:"+(parseFloat(i.value)+parseFloat(i.value));
  }
  function f() {

    //判断用户输入的内容是否是数值 如果是NaN代表不是数值
    if (isNaN(i.value)){
      d.innerText = "输入错误!";
      return;
    }
    d.innerText = "结果:"+i.value*i.value;
  }
</script>
</body>
</html>